The brain region of the embryonic central nervous system expands markedly once neurulation is completed and the neural tube becomes a closed, fluid filled system. Expansion occurs at the same time that the tube is sealed at both ends as a result of closure of the rostral neuropore and occlusion of the spinal cord. This rapid expansion is directly controlled by intra- luminal pressure generated by neural tube fluid. Several hypotheses examining highly-related phenomena of brain growth in chick embryos will be tested. These hypotheses are: (1) precocious closure of the neural tube will result in precocious enlargement of the brain; (2) initiation of brain expansion requires a critical intra-luminal pressure (ILP) maintained by a critical amount of fluid within the cavity; (3) normal growth of the neuroepithelium during rapid brain expansion requires a critical ILP; (4) recovery of the critical ILP to support normal tissue growth and cavity expansion, thus brain enlargement, requires active transport of fluid across the neuroepithelium; (5) the neuroepithelium secretes fluid via active transport once the neural tube is sealed anteriorly and posteriorly; (6) the protein composition of neural tube fluid is identical to that of the blood plasma during the period of rapid brain expansion; and (7) active transport of fluid across the neuroepithelium is exclusively via Na+/K+- ATPase pumps. The spinal cord of chick embryos will be occluded experimentally during the stage in development immediately prior to when occlusion occurs normally. Preliminary data suggest that the brain in these embryos expands earlier and thirty times more than controls during this period. Experiments are proposed to determine the intra-luminal pressure and transepithelial potential prior to and during expansion of the brain; to determine when the neuroepithelium first secretes fluid sufficient to initiate the rapid expansion of isolated neuroepithelial mini-vesicles; to determine the mechanism of fluid transport across the apical surface of the neuroepithelium using neuroepithelial mini-vesicles, whole brain vesicles, and whole embryo culture; and to determine the effects of pharmacological agents (having known effects on epithelial transport) on intra-luminal pressure, brain growth, and fluid transport. The major focus of this study will be to elucidate the relatedness of fluid accumulation, expansion, intra-luminal pressure, and the mechanism by which fluid is transported across the neuroepithelium. Experiments are proposed to distinguish whether fluid transport across the neuroepithelium is active via Na+/K+- ATPase pumps or passive via osmosis.